Method and system for defence against surface-to-air missiles

ABSTRACT

A method and a system for defense against surface-to-air missiles (Manpads), which represent a threat to military and civil aircraft during takeoff and landing. For missile defense, the missile is jammed by irradiating it with electromagnetic jamming radiation. This jamming radiation may be constituted of continuous-wave irradiation or frequency packets with a defined pulse repetition rate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and also to a system, which provide for a defense against surface-to-air missiles which represent a threat to military and civil aircraft during takeoff and landing.

2. Discussion of the Prior Art

Military and civil aircraft can be attacked by surface-to-air missiles (Manpads=Man Portable Air Defense Systems) during takeoff and landing. Manpads such as these are available even to terrorist groups throughout the world and in consequence represent an increasingly serious threat. These surface-to-air missiles are generally of a relatively old type and at the moment are generally still equipped with analogue electronics.

SUMMARY OF THE INVENTION

The invention is based on the object of providing a method and a system of the type mentioned initially which, using simple means, is suitable for defense against surface-to-air missiles (=Manpads), which represent a threat to military and civil aircraft during takeoff and landing.

With regard to the method, this object is achieved by a jamming of the missile through irradiating it with jamming radiation. Preferred embodiments and developments of the method according to the invention are further characterized in the dependent claims. The object on which the invention is based is achieved, with regard to the system, by providing an Arbitrary Waveform Generator (AWG) and a number of parallel-connected individual modules, which each have a phase shifter, an amplifier downstream of the phase shifter, an antenna downstream of the amplifier, and a phase detector which is associated with the phase shifter. Preferred embodiments and developments of the system according to the invention are detailed in various dependent claims. The object on which the invention is based can also be achieved by the system, which provides for a number of parallel-connected individual modules, which each have an AWG with an integrated multi-frequency phase shifter, an amplifier downstream therefrom, and an antenna downstream of the amplifier, with the AWGs being synchronized via a master lock.

The applicants have derived interaction data for the capability to influence various Manpads by irradiation with electromagnetic jamming radiation. The Manpads investigated exhibit different sensitivity profiles over the respectively injected frequency of the electromagnetic jamming radiation. Jamming voltages with different amplitudes are produced on the sensitive signal lines of the analogue control electronics of the Manpads, as a function of the frequency of the jamming radiation. In addition, pronounced resonance effects occur. In this case, it has also been found that the resonant frequencies vary to a certain extent as a function of the angle of incidence of the radiation (AOI=angle of incidence) of the jamming radiation on the surface-to-air missile.

In addition, relatively old missiles with analogue control electronics such as these also have internal operating frequencies. If the jamming radiation that is suitable for ideal injection is additionally modulated or clocked with a frequency which corresponds to the appropriate operating frequency, then it is probable that the homing head of the missile to be defended against will lose the target.

A required pulse repetition frequency can also have superimposed on it the frequency of a rolling movement of the missile to be defended against.

The behaviour of the homing head and thus the trajectory of the missile to be defended against can be simulated. Appropriate simulation tools have been developed for various Manpads by the applicants. This makes it possible to verify the effectiveness of electromagnetic jamming irradiation with optimized parameters.

Pulsed irradiation with the ideal clock frequency results in the field strength required for missile defense and attack rising as the pulse duration becomes shorter, that is to say the field strength is inversely dependent on the pulse duration. Optimization is in consequence possible in terms of the respective minimum required field strength and the shortest possible pulse length.

In addition to the dependency of the resonant frequencies on the angle of incidence (AOI), the required field strengths and pulse lengths differ for different missiles, in the same way as the required clock frequencies.

A missile to be defended against or a plurality of—also different—attacking missiles may, according to the invention, have electromagnetic jamming radiation applied to it or them after detection, by means of phase control which can be directed and which it may be possible to slave. Slaving may comprise mechanical or electronic slaving by means of phase control (beam steering).

The electromagnetic jamming radiation may comprise radio-frequency waves (RF) or microwaves (MW). The waveform emitted from at least one jamming radiation source, in terms of its frequency, pulse length, pulse amplitude, that is to say electrical field strength, clocking etc., and its time profile, are such that the operation of the at least one attacking missile is permanently jammed, so that it can no longer carry out its mission.

According to the invention, a missile can be jammed and thus defended against by means of continuous-wave irradiation (CW) at a suitable frequency f. When a Manpad is irradiated with a known frequency, which has previously been determined in laboratory experiments and is ideal for the injection of jamming, the missile can be deflected from its trajectory. In general, one advantage of continuous-wave irradiation is that a relatively low field-strength amplitude is required at the target. This allows very great ranges to be achieved with a given transmission power and antenna configuration. However, CW irradiation requires a relatively high power level. Furthermore, one specific fixed frequency generally allows only one specific missile to be defended against. This means that it may possibly be necessary to identify the attacking missile. Furthermore, the ideal injection frequency is generally shifted for different irradiation angles, that is to say angles of incidence (AOI), which may result in a reduction in the jamming or defensive effect.

The jamming radiation frequency can be tuned over a predetermined frequency range according to the invention, as well, in order to compensate for any discrepancies from the optimum jamming frequency which may, for example, be a result of missile-specific and/or trajectory-specific constraints. This involves somewhat more technological complexity and the need to scan the frequency range within a relatively narrow time interval, in order to ensure the appropriate effect at the missile to be defended against. Another option according to the invention is to use frequency packets at the frequency f, with a suitable defined pulse repetition rate, as the electromagnetic jamming radiation. This means that, as an alternative to the continuous-wave irradiation of a missile to be defended against, as mentioned above, it is also possible to irradiate the missile in a clocked form with short pulses at the optimum injection frequency f₁. The field strength E₁ at the target, that is to say at the missile to be defended against, that is required to defend against that missile generally increases with short pulses. In this case, an optimum can be found at which the necessary field strength still rises very little in comparison to CW irradiation in order to achieve the same defensive effect. The clocking, that is to say the repetition rate of the pulses, and the injection frequency, must be chosen as appropriate for the missile to be defended against.

The jamming of, that is to say defense against, a missile by irradiation with frequency packets at the frequency fi and with a suitable pulse repetition rate has the advantage over CW irradiation that it results in a reduced mean power requirement, with appropriate optimization, for the same defensive effect. On the other hand, a somewhat higher peak field strength is required at the target, and a somewhat reduced range is possible when the power limit of the radiation source is reached.

Continuous-wave irradiation, that is to say CW irradiation, and the use of frequency packet irradiation for jamming the missile do not overcome the relationship between the resonant frequencies of the missile and the angle of incidence. Since the angle of incidence cannot easily be determined, it is proposed, for example, to emit different frequency packets immediately successively in each pulse. These frequency packets are selected as appropriate from previous injection experiments using different angles of incidence, in order to obtain a good cross section for injection in at least one case, that is to say for at least one frequency. This means that, instead of having to use frequency packets at one specific constant injection frequency, it may be advantageous to use frequency packets with varying injection frequencies. These changes to the injection frequencies may be in steps or continuously. A so-called frequency sweep is therefore also suitable, as an alternative to discrete frequencies. This frequency sweep can also be chosen as appropriate on the basis of the interaction data. The bandwidth and the sweep rate are kept relatively narrow and low, respectively, in order to carry out effective injection over an adequate irradiation time, with respect to the width of the injection resonance. This is because, if the sweep were too fast, the required field strength for jamming would rise analogously to very short pulse durations. If a missile to be defended against is irradiated with pulsed frequency packets, that is to say pulse packets, which each comprise a frequency sweep, this results in the advantage that, with an appropriate design, it is not necessary to know the angle of incidence of the missile. However, the mean power requirement increases.

According to the invention, it is also possible to use frequency packets, which differ from one another and have pulse repetition rates that differ from one another as the electromagnetic jamming radiation, with the time windows, which are governed by the pulse repetition rate of a first frequency packet, being used for at least one-second frequency packet. This allows a plurality of missiles, or different missiles, to be jammed by time-division multiplexing of different missile-specific frequency packets. If all the frequencies for one type of missile or for groups of missiles are covered for different angles of incidence (AOI), then pulsing at a first pulse repetition rate results in time gaps which can be used for further missiles. These time gaps are filled with at least one further frequency packet. The number of different missiles to be attacked using different parameters is in this case limited only by the maximum time window that is available for attack purposes. This method variant has the advantage that a plurality of missile types can be attacked, without any need to identify the different missile types. This method according to the invention requires a high mean power level, however; maximum utilization of the time gaps results in a power requirement which corresponds approximately to the power requirement for continuous-wave irradiation, that has been described further above.

According to the invention, it is also possible to use parallel additive frequency packets with frequencies which differ from one another as the electromagnetic jamming radiation. This allows a plurality of missiles, or different missiles, to be jammed by parallel additive emission of a plurality of different missile-specific frequency packets. This is because, in a similar manner to the method according to the invention described first of all, it is also possible to emit the determined frequency packets, in an optimally specific form for the missile, in parallel and additively. This has the advantage that it avoids any restriction to time gaps or to the available time window. In principle, any desired number of different missiles can therefore be irradiated. In this case, additive frequency mixing can be used to attack a plurality of missile groups at the same time. The power required by an amplifier increases in this case, of course, since the maximum amplitude, that is to say the field strength, may be two or more times the two individual amplitudes or plurality of individual amplitudes. However, this procedure has the advantages that a plurality of missile types can be attacked, without any need to identify the different missile types. A further advantage is that there is no restriction to the number of frequency patterns resulting from the subdivision of time gaps.

The system according to the invention for carrying out the method according to the invention may be characterized in that an Arbitrary Waveform Generator (AWG) and a number of parallel-connected individual modules are provided, which each have a phase shifter, an amplifier downstream from the phase shifter, an antenna downstream from the amplifier, and a phase detector associated with the phase shifter. It is therefore possible to emit high-power electromagnetic waves which, by means of suitable emitted signal forms, cause missions of one or more missiles to be rendered ineffective at the same time, by jamming its or their control and steering circuit electronics. Since the individual amplifier systems have a restricted maximum power, addition of a plurality of amplifier systems in the correct phase is desirable according to the invention. The amplifiers and the antennas in the individual modules normally have a frequency-dependent and amplitude-dependent phase shift between the input and the output, which can also vary between the respective amplifiers in the individual modules. If amplifier-antenna systems are cascaded in order to influence Manpads, it is, however, necessary to ensure phase synchronization at the respective antenna outputs. Phase control is therefore provided between the respective amplifier input and the antenna.

Connecting the individual modules in parallel allows more power to be emitted, and a higher gain. The necessary phase synchronicity is achieved by a phase detector/phase shifter structure, which regulates the phase between the antenna output signal and the AWG output to be the same. For this purpose, appropriate signals can be tapped off directly from the antenna or from the respectively associated amplifier.

Different frequency bursts are emitted sequentially in the AWG, in time with the critical pulse repetition rate. This allows adaptation of the direction-dependent frequency selectivity of the target, that is to say Manpad. The limit to the pulse repetition rate, the pulse length and the number of frequencies is defined by the finite time window mentioned above.

This system according to the invention has the advantage that only a single AWG—or DDS synchronizer—is required. The individual modules, with feedback, guarantee that the phases are the same, thus advantageously allowing simple parallel connection and coupling to the AWG.

If an electrically controlled directional effect is desired, this can be achieved by additional phase shifters, that is to say delay elements, downstream from the AWG.

The need for sequential arrangement of the sine-wave functions in the AWG is based on the fact that the phase detector and phase shifter mentioned above are effective for only one sine/time function. An extension to the system is based on frequency separation of the antenna measurement signal and of the AWG signal by means of bandpass filters. In this case, phase detection and correction at the phase shifter are carried out for each frequency that is filtered out in this way. This also allows simultaneous addition of the frequencies to be emitted, without any restriction resulting from the maximum available time window.

The system according to the invention can also be implemented in such a way that a number of parallel-connected individual modules are provided, which each have an AWG with an integrated, multi-frequency phase shifter, an amplifier downstream from it and an antenna downstream from the amplifier, with the AWGs being synchronized via a master clock. If the system according to the invention is designed in this way, the amplifiers and antenna system likewise have a frequency-dependent and amplitude-dependent phase shift between the input and the output. This phase shift can also vary between the amplifiers. When amplifier-antenna systems are cascaded in order to influence Manpads, the phases at each of the antenna outputs must, however, be the same. Phase control, that is to say phase correction, is for this purpose provided in a digital form between an absolute reference phase for all of the AWGs and the respective antenna output, in the AWG.

Connecting the individual modules in parallel allows more power to be emitted and a higher gain. The necessary phase synchronicity is achieved by a multi-frequency phase adaptation in completely digital form, controlling the phase between the antenna output signal and the internal AWG frequency to be the same for all of the frequencies which are emitted at the same time. In this case, all of the AWGs are synchronized via a master clock. Frequency bursts at different frequencies are added in the AWG, and are emitted in time with the critical pulse repetition rate. This allows adaptation of the direction-dependent frequency selectivity of the target, while allowing different targets to be attacked at the same time. The only limit to the number of different frequencies is the maximum amplifier output power.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, features and advantages of the invention will be evident from the following description of the figures, in which:

FIG. 1 shows an energy-time diagram for continuous-wave irradiation;

FIG. 2 shows an energy-time diagram for irradiation of a missile with energy pulses;

FIG. 3 shows an energy-time diagram of irradiation with pulsed energy packets, which are each composed of different discrete frequencies;

FIG. 4 shows an energy-time diagram for irradiation with pulsed energy packets, which each comprise a frequency sweep;

FIG. 5 shows an energy-time diagram in order to illustrate sequential irradiation with parameters, whose frequency and pulse repetition rate are matched, for different missiles or missile groups;

FIG. 6 shows an energy-time diagram in order to illustrate irradiation with parameters, whose frequency and pulse repetition rate are matched, for different missiles or missile groups in the parallel additive mode;

FIG. 7 shows one embodiment of the system according to the invention with an AWG;

FIG. 8 shows an extension to the implementation capability of the system as shown in FIG. 7, illustrating only one individual module; and

FIG. 9 shows another embodiment of the system according to the invention, with individual modules, which each have an AWG, connected in parallel.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an energy-time diagram of continuous-wave irradiation of a Manpad at a frequency f₁, which is ideal for one specific missile to be defended against, and with the energy E₀. When a missile to be defended against is irradiated at a known frequency f₁, which has been determined in advance in laboratory experiments and is ideal for the injection of jamming, the missile can be deflected from its trajectory. Continuous-wave irradiation such as this generally requires the lowest field-strength amplitude E₀ at the target. This allows the maximum ranges to be achieved for a given transmission power and antenna configuration.

FIG. 2 shows an energy-time diagram of the jamming of a missile to be defended against by irradiation with frequency packets at the frequency f₁ and with a suitable pulse repetition rate Δt. In this case, Δt=1/f_(PPR), where f_(PRR) is the respective pulsing, which is chosen on a missile-specific basis, in the same way as the injection frequency f₁. The field strength E₁ at the target required for defense against a missile generally increases as the pulses become shorter; however, an optimum can be found at which the required field strength E₁ still rises very little in comparison to the field strength E₀ for continuous-wave irradiation (see FIG. 1), in order to achieve the same effect.

The continuous-wave irradiation as shown in FIG. 1 and jamming of the missile by irradiation with frequency packets at the frequency f₁ and with a suitable pulse repetition rate Δt does not overcome the dependency of the resonant frequencies of the respective missile on the angle of incidence. Since the angle of incidence cannot be determined easily, it is possible, for example, to emit different frequency packets immediately successively in each clock cycle Δt. These frequency packets are chosen appropriately from previous injection experiments using different angles of incidence, in order to obtain a good cross section for injection in at least one case—for at least one frequency. This is illustrated in FIG. 3, which shows an energy-time diagram for irradiation with pulsed packets, which are each composed of different, successive, discrete frequencies f₁₋, f₁, f₁₊. The pulse repetition rate Δt is the same as that shown in FIG. 2.

FIG. 4 shows an energy-time diagram of the irradiation of a missile to be defended against with pulsed packets which each comprise a frequency sweep f¹⁻ . . . f₁₊. This means that a frequency sweep, that is to say continuous frequency variation, is possible as an alternative to switched discrete frequencies f¹⁻, f₁, f₁₊. This frequency sweep is chosen appropriately, on the basis of the corresponding interaction data.

FIG. 5 shows an energy-time diagram for sequential irradiation with parameters, whose frequency and pulse repetition rate are matched, for different missiles or missile groups to be defended against. If all of the frequencies for one type of missile—or missile group—are covered for different irradiation angles (AOI), then pulsing at a specific pulse repetition rate Δt₁ results in time gaps in the corresponding time period, and these can be used for further missiles. These time gaps between the frequency packets f¹⁻, f₁, f₁₊ are filled with further frequency packets f₂. The number of missiles which can be attacked with different parameters is limited only by the said maximum time window that is available for attack purposes.

The pulse repetition rate Δt₂ of the frequency packets f₂ is governed by the pulse repetition rate Δt₁.

FIG. 6 shows irradiation with parameters, whose frequency and pulse repetition rate are matched, for different missiles or missile groups in the parallel-additive mode. This avoids a restriction to gaps in the time period, that is to say to an available time window. In principle, any desired number of different missiles can be irradiated, in order to defend against them. FIG. 6 shows an exemplary embodiment for two missile groups, for which the necessary frequency packets f₁ and f₂ overlap in time. Nevertheless, additive frequency mixing allows both missile groups to be attacked at the same time.

FIG. 7 illustrates a system 10 according to the invention for defense against surface-to-air missiles (Manpads) which represent a threat to military and civil aircraft during takeoff and landing. The system 10 has an Arbitrary Waveform Generator (AWG) 12 and a number of parallel-connected individual modules 14. Each individual module 14 has a phase shifter 16, an amplifier 18 downstream from the phase shifter 16, and an antenna 20 downstream from the amplifier 18. Each phase shifter 16 has an associated phase detector 22. This allows a corresponding number of amplifier systems to be added in the correct phase, since the individual amplifier systems each have a limited maximum power. The amplifiers 18 and the antennas 20 normally have a frequency-dependent and amplitude-dependent phase shift between the input and the output, and this phase shift may also vary between the individual amplifiers 18. However, if amplifier-antenna systems are cascaded in order to influence Manpads, that is to say to defend against them, it is necessary to ensure phase synchronization at each of the antenna outputs. Phase control is carried out for this purpose between the respective amplifier input and the antenna 20.

Connecting the individual modules 14 in parallel allows more power to be emitted, and a higher antenna gain. The required phase synchronicity is achieved by the phase detector/phase shifter structure which controls the phase between the antenna output signal and the output of the AWG 12 to be the same. For this purpose, the appropriate signals can be tapped off, for example, directly from the respective antenna 20—or from the corresponding amplifier 18.

Different frequency bursts are emitted sequentially in time with the critical pulse repetition rate Δt in the AWG 12. This allows adaptation of the direction-dependent frequency selectivity of a Manpad.

The system 10 shown in FIG. 7 requires only a single AWG 12—or DDS synthesizer.

The individual modules 14 are each connected in parallel with one another, by means of a parallel phase shifter 28. The parallel phase shifters 28 are operatively connected to one another by means of a joint phased array controller 30, which processes frequency information from the AWG 12.

FIG. 8 illustrates an extension to the system shown in FIG. 7. In this embodiment, an AWG 12 and a number of parallel-connected individual modules 14 are provided, only one of which is shown in FIG. 8. Each individual module 14 has a pair of phase shifters 16 in parallel with one another, a phase detector 22 associated with the respective phase shifter 16, an amplifier 18 downstream from the two phase shifters 16 and an antenna 20 downstream from the amplifier 18. Each of the phase shifters 16 and the respectively associated phase detector 22 have an associated bandpass filter 24.

The need for sequential arrangement of the sine-wave functions in the AWG 12 is based on the fact that the phase detector 22 and the phase shifter 16 are effected for only one sine-time function. The extension to the system shown in FIG. 8 is based on frequency separation of the antenna measurement signal and the AWG signal by means of bandpass filters 24, with only the phase detection and correction at the phase shifter 16 being carried out for each frequency which is filtered out in this way. This allows simultaneous addition of the frequencies to be emitted, and avoids any restriction to the maximum available time window. This covers the application illustrated in FIG. 6.

FIG. 9 shows an embodiment of the system 10 in which a number of parallel-connected individual modules 14 are provided and each have an AWG 12 with an integrated multi-frequency phase shifter, an amplifier 18 downstream from the respective AWG 12, and an antenna 20 downstream from the respective amplifier 18, with the AWGs 12 being synchronized via a master clock 26.

Connecting the individual modules 14 in parallel as shown in FIG. 9 allows more power to be emitted and a correspondingly higher antenna gain. The required phase synchronicity is achieved by multi-frequency phase adaptation provided in an entirely digital form, which controls the phase, for all of the frequencies emitted at the same time, between the output signal of the respective antenna 20 and the internal AWG frequency, to be the same. Frequency bursts at different frequencies are added in the AWG 12 and are emitted in time with the critical pulse repetition rate Δt. This allows adaptation of the direction-dependent frequency selectivity of the missile to be defended against, while allowing different targets, that is to say missiles, to be attacked at the same time.

LIST OF REFERENCE SYMBOLS

-   10 System -   12 AWG/Arbitrary Waveform Generator (of 10) -   14 Individual module (of 10) -   16 Phase shifter (of 14) -   18 Amplifier (of 14) -   20 Antenna (of 14) -   22 Phase detector (for 16) -   24 Bandpass filter (of 14) -   26 Master clock (of 10) -   28 Parallel phase shifter -   30 Phased array controller 

1. A method for defense against surface-to-air missiles (Manpads=Man Portable Air Defense Systems) which represent a threat to military and civil aircraft during takeoff and landing, wherein the missile is jammed by irradiating it with electromagnetic jamming radiation.
 2. A method according to claim 1, wherein the electromagnetic jamming radiation utilizes microwave radiation or radio-frequency radiation as the electromagnetic jamming radiation.
 3. A method according to claim 1 or 2, wherein the electromagnetic jamming radiation utilizes continuous-wave radiation.
 4. A method according to claim 3, wherein the continuous-wave irradiation uses a constant jamming radiation frequency.
 5. A method according to claim 3, wherein the continuous-wave irradiation is tuned over a predetermined jamming radiation frequency range.
 6. A method according to claim 1 or 2, wherein frequency packets with a defined pulse repetition rate are used as the electromagnetic jamming radiation.
 7. A method according to claim 6, wherein there are utilized said frequency packets with a specific constant injection frequency.
 8. A method according to claim 6, wherein there are utilized said frequency packets with injection frequencies that are varied in steps.
 9. A method according to claim 6, wherein there are utilized said frequency packets with injection frequencies that are varied continuously.
 10. A method according to claim 1 or 2, wherein there are utilized said frequency packets which differ from one another and have pulse repetition rates that differ from one another as the electromagnetic jamming radiation, with time windows, which are governed by the pulse repetition rate of a first frequency packet, being used for at least one second frequency packet.
 11. A method according to claim 1 or 2, wherein there are utilized parallel additive frequency packets with frequencies which differ from one another as the electromagnetic jamming radiation.
 12. A method according to claim 11, wherein there are utilized frequency packets with different pulse repetition rates.
 13. A system for defense against surface-to-air missiles (Manpads=Man Portable Air Defense Systems) which represent a threat to military and civil aircraft during takeoff and landing, including an Arbitrary Waveform Generator (AWG) (12) and a number of parallel-connected individual modules (14), which each have a phase shifter (16), an amplifier (18) downstream of the phase shifter, an antenna (20) downstream of the amplifier, and a phase detector (22), which is associated with the phase shifter (16).
 14. A system according to claim 13, wherein the individual modules (14) are each connected in parallel with one another by a parallel phase shifter (28).
 15. A system according to claim 13, wherein the parallel-phase shifters (28) are operatively connected to one another by a joint phased array controller (30) which processes frequency information from the AWG (12).
 16. A system according to claim 13, wherein there are provided an AWG (12) and a number of parallel-connected individual modules (14), each having a pair of mutually parallel phase shifters (16), a phase detector (22), which is associated with a respective said phase shifter, an amplifier (18) downstream of the two phase shifters (16), and an antenna (20) downstream of the amplifier, with the respective phase shifter (16) and the therewith associated phase detector (22) each having an associated bandpass filter (24).
 17. A system for defense against surface-to-air missiles (Manpads=Man Portable Air Defense System) which represent a threat to military and civil aircraft during takeoff and landing, wherein there are provided a number of parallel-connected individual modules (14), which each have an AWG (12) with an integrated, multi-frequency phase shifter, an amplifier (18) downstream therefrom, and an antenna (20) downstream of the amplifier (18), with the AWGs (12) being synchronized via a master clock (26). 